This invention relates generally to audio systems. More particularly, this invention relates to a loudspeaker system that utilizes pressure feedback control to improve bandwidth and reduce distortion.
Modem recording techniques and contemporary playback systems (e.g. Compact Disc players) are capable of reproducing signals in the extremes of the audible spectrum (from 20 Hz to 20 kHz). Since the weakest link in a well designed audio system is typically its loudspeaker system, rigid requirements are placed on the loudspeaker""s design. An important part of the audio spectrum is the sub-bass frequency region (i.e., between 20 and 100 Hz). The audio industry is endeavoring to achieve a flatter amplitude and phase response and lower distortion in the sub-bass frequency region. Loudspeakers that operate in the sub-bass frequency region are frequently referred to as woofers or subwoofers.
Faithful reproduction of sub-bass signals by conventional subwoofers requires sophisticated electromechanical design techniques, advanced materials, and tight tolerances, necessitating added manufacturing costs. However, even expensive subwoofer systems are limited by the inherent dynamics of low-frequency reproduction and become highly nonlinear well before reaching their maximum acoustic output.
Several manufacturers of audio equipment have built signal processors that merely equalize the signal into the subwoofer in order to flatten the subwoofer""s frequency response. Even though some of these companies describe the method as xe2x80x9cServo Controlxe2x80x9d, it isn""t. By definition, servo control necessitates feedback signals from a controllable system. While the equalizing method has the advantage of simplicity (a sensor is not necessary), it cannot compensate for variations in the subwoofer""s behavior (e.g. aging, effects of the environment on the subwoofer due to changes in acoustic venues, temperatures, air densities, etc.).
Some high-end loudspeakers utilize subwoofers with feedback control derived from accelerometers attached to the voice-coil. This approach is undesirable for two reasons. First, the accelerometer adds additional moving mass to the system, thereby reducing efficiency. Second, and most important, the invasive nature precludes its use in low to mid priced systems (i.e. those that dominate the market) as well as the market of consumers that already own subwoofers and are looking for xe2x80x9cadd onxe2x80x9d devices to improve performance.
In view of the foregoing, it would be highly desirable to provide an improved loudspeaker system to achieve a flatter amplitude response and lower distortion in the sub-bass frequency region. Ideally, such a system would be immune from the effects of aging and environmental conditions, such as acoustic venues, temperatures, and air densities.
A sound producing system includes a loudspeaker to generate an audible signal. A transducer generates a pressure feedback signal from the audible signal. A controller produces a controlled signal for application to the loudspeaker. The controller derives the controlled signal from the pressure feedback signal and an audio input signal. The controlled signal improves the Sound Pressure Level (SPL) frequency response and reduces distortion within a selected bandwidth of the loudspeaker.
The system of the invention extends the performance limits in high fidelity loudspeaker systems. In particular, the invention""s analysis and control techniques achieve a flatter amplitude response (both magnitude and phase) and lower distortion in the low-frequency, sub-bass region. The invention exploits feedback control to minimize the effects of system aging and environmental conditions, such as acoustic venues, temperatures, and air densities. The feedback scheme of the invention is based upon pressure signals from a microphone. Thus, unlike prior art accelerometer-based motion feedback schemes, the system of the invention does not add mass to the acoustic elements of the system and is readily retrofitted into existing systems. Advantageously, the system of the invention allows for a reduced loudspeaker size. The invention can be readily exploited in consumer devices, such as high fidelity car audio systems and home theater applications. Professional uses for the invention include movie theaters, recording studios, and concert systems.